The present invention relates to a scintillation camera apparatus which is employed in medical diagnosis, and more particularly, to the processing of radioisotope distribution imaging signals in a scintillation camera apparatus.
A scintillation camera apparatus for use in medical diagnosis comprises a scintillation camera (i.e., a gamma camera) and a data-processing device. The camera detects the distribution of a radioisotope (RI) which has been injected to a subject under examination and which is concentrated within a specific organ of the subject. The data-processing device processes data provided by the camera, to image the RI distribution within a region of interest of the subject. The scintillation camera is provided with a collimator, a scintillation crystal, and photomultipliers. It is used as a two-dimensional detector for detecting gamma rays radiated from inside the subject.
In this scintillation camera apparatus, the gamma rays radiated from inside the subject are detected as events of scintillation, by means of the scintillation crystal and photomultipliers. Gamma-ray detection signals are applied to a position-calculating circuit, as well as to an energy signal-generating circuit. The position-calculating circuit produces position signals X and Y representing the position of the generation of gamma rays. The energy signal-generating circuit generates an energy signal Z representing the intensity of detected gamma rays. Position signals X and Y and energy signal Z are used for imaging the RI distribution within the subject. More specifically, the scintillation camera also generates an energy-discriminating signal Z.sub.PHA. When the magnitude of the Z.sub.PHA signal falls within a window (range) of interest, an unblank signal (hereinafter referred to as "UNB signal") is genertated. Position signals X and Y, energy signal Z, and the UNB signal together enable the forming of an RI distribution image. The digitization of signals X, Y, and Z is enabled by the UNB signal. When the UNB signal is generated, the position defined by signals X and Y is imaged as a source of gamma rays. Each time an event of gamma-ray scintillation is detected, a +1 count is accumulated at that memory location of an image acquisition memory which is addressed by position signals X and Y. Hence, the RI distribution can be imaged on a display, based on the event information, stored in the image memory, which represents the generation position, and the intensity of the gamma rays.
Pulse-height analysis (PHA) is used in a scintillation camera apparatus in order to determine whether or not a Z.sub.PHA signal falls within a specified window of interest. This analysis is performed within a predetermined sampling period.
Two or more events of scintillation may take place almost simultaneously on the scintillation crystal, irrespective of the scintillation positions. If such is the case, the waveform of the Z.sub.PHA signal will have two or more peaks during the sampling period. This type of the Z.sub.PHA signal waveform is referred to as a "pileup waveform." When a conventional pulse height analysis technique is employed, a UNB signal is generated in response to only the first peak of the pileup waveform of the Z.sub.PHA signal. In other words, another peak, which has been produced at almost the same time as the first peak, cannot be detected or identified. Consequently, the scintillation positions of gamma rays may be calculated incorrectly.
This calculation error can be observed more noticeably at the center portion of the two-dimensional detector, than at the peripheral portions thereof. The higher the count rate, or the more frequently the scintillation events occur, the greater the deterioration in the uniformity profile of the detector. In particular, the uniformity profile is not flat; it is higher at the center than at the peripheral portions.